Here's how I do it.
I use water-based paints and the most important thing is to use a slow drying medium! Otherwise it will not work...
The smallest brush I'll use is a #3.
I will paint a "Bay color" horse, so as the base color I use light rust.
It is mixed with slow drying medium and water to make a glaze.
It should NOT be laid as a wash, but in thin layers without pooling in the recesses.
Four layers of glaze.
Here I have added a layer of medium rust to warm up the color a bit.
Mix a lighter version of the base color by using a little white and apply on the lightest surfaces.
I glaze the legs with medium rust.
Here I have painted all the details and the legs with dark rust mixed with black.
All of the details are highlighted with lighter versions of their base color.
the eyes are painted dark brown.
The metal fittings are painted in aluminum.
Mix a dark brown wash and gently apply it around all the details to highlight them.
Finished!